Hydroxy functional polydiene polymers are well known. U.S. Pat. No. 5,393,843 discloses that formulations containing these polymers, a melamine resin, and an acid catalyst can be cured by baking under normal bake conditions. This same patent also discloses that these polymers can be mixed with isocyanates to yield compositions which cure at ambient temperature. However, these compositions based on hydroxy functional polydiene polymers cured with a crosslinking agent are usually fairly soft and, although they can be very useful in some applications, they have limited adhesion. Attempts to increase hardness and adhesion by increasing crosslink density by simply mixing these components with a reinforcing agent, such as a low molecular weight diol or triol, and higher levels of crosslinking agent were unsuccessful because the reinforcers and crosslinkers are relatively polar and so are incompatible with the relatively nonpolar polydiene polymers. Incompatibility of the components leads to poor properties, such as gloss, in the cured composition or, even worse, the compositions may phase separate upon storage prior to cure.
One potential use for these compositions is for direct application to an acceptable substrate such as a plastic automotive exterior part. They are also highly useful in coatings for use on substrates which have already been coated with a primer coating. An example of this type of application is in basecoat coatings (basecoats) in the automotive industry. The basecoat contains pigment and is the actual "paint" which goes onto the car. A clear coat is usually applied over the basecoat to preserve the color. One primary substrate to which the basecoats will be applied is steel which has been primed with an epoxy resin based primer which was applied by the cathodic electrodeposition (CED) process. Another substrate which could be used is the epoxy primed steel which has also been coated with a primer/surfacer which is usually based on a polyester resin.
This invention presents novel compatible coating compositions prepared from these components which exhibit excellent adhesion to primed and unprimed substrates and provides a process for the preparation of these novel compositions. This invention thus provides a composition which may be used to "paint" the entire exterior of an automobile.